Films composed of polyesters within the thickness range specified are sufficiently well known. However, a disadvantage of such polyester films is their hydrolysis tendency, especially at temperatures above the glass transition temperature of the particular polyester. In this context, the hydrolysis tendency is understood to mean the property of the polyester of being degraded hydrolytically under moist conditions, which is noticeable, for example, by a reduction in the IV or SV. This is a limiting factor for the use of polyester films especially in applications with relatively high thermal stress, such as in film capacitors, cable sheathing, ribbon cables, engine protection films, but also in long-term applications such as in glazing and outdoor applications, and especially in the backside laminate of solar modules.
The hydrolysis tendency is particularly marked in the case of aliphatic polyesters, but also in the case of aromatic polyesters such as PBT and PET. When the hydrolysis tendency of PET becomes too great for the application, it is necessary to revert to the more hydrolysis-stable PEN or even to other polymers, for example polyetherimides or polyimides. However, these are significantly more expensive than PET and are therefore frequently no solution for economic reasons.
It has therefore already been proposed to improve the hydrolysis stability of polyester films through the incorporation of hydrolysis stabilizers.
More hydrolysis-resistant polyester raw materials which are obtained through use of carbodiimides, and fibers and films produced therefrom, are known (U.S. Pat. No. 5,885,709, EP-A-0 838 500, CH-A-621 135). Films which are produced from such raw materials, however, tend both in production and in later use to outgas isocyanates and other mucosa-irritant or harmful by-products and degradation products. This is a much greater problem in flat structures such as films with a large surface area than, for example, in injection moldings.
Polyester films with epoxidized vegetable oils as stabilizers are described in EP-A-1 634 914 and EP-A-1 842 871. The toxic degradation products which are typical of carbodiimides do not occur here, the incorporation into the polyester matrix is good given suitable selection of the oils, and there is good hydrolysis stabilization of the films. Disadvantages which occur in application are, however, recurring production intervals with an extreme tendency to gel formation in production. When the gel level becomes too high, this results in breakoffs, and production of the film becomes impossible for a time. Production then has to be stopped and the extrusion has to be cleaned.
Moreover, the fatty acid esters described there, owing to their quite low molecular weight, have a high vapor pressure under the polyester production conditions. This leads firstly to the outgassing thereof during production with corresponding odor nuisance and fatty acid ester condensates on production plant parts, and leads additionally to increased migration of these molecules out of the film, which can lead in downstream processing operations to adhesion problems in lamination, metallization, etc. During production, these compounds additionally get into the extruder vacuum (in the case of use of twin-screw/multiscrew extruders (and are thus no longer available in the film at all as a hydrolysis stabilizer.